(1). Field of the Invention
The present invention relates to a wire feeding device, particularly relating to a wire feeding device for a multi-wire saw, in which a single wire, one end of which is connected to a feed reel and the other end is connected to a collecting reel, is spirally wound between at least two work rollers arranged in parallel, a fixed distance apart, so that the turns of the spirally wound wire are separated at predetermined intervals.
(2). Description of the Prior Art
FIGS. 1 and 2 shows an example of the multi-wire saw. FIG. 1 is a front view of the apparatus and FIG. 2 is an enlarged top view showing work rollers of the apparatus.
The multi-wire saw includes at least two (just two, in this example) cylindrical work rollers 1 and 2 on which spiral grooves 11 and 21 are formed for tensioning a wire 3. The rollers 1 and 2 are arranged so that the central axes of each roller is in a horizontal direction and in parallel with each other. The single wire 3 is wound around the spiral grooves 11 and 21 of the two work rollers 1 and 2, to form a multiple number of stretched sections of the wire 3 (see FIG. 2). The wire 3 is connected at one end to a feed reel 4 and the other end is connected to a collecting reel 5.
That is, while being tensioned by right and left weights 9a and 9b , the wire 3 is stretched via fixed pulley wheels 6, 6 . . . , running pulley wheels 7a, 7b, 8a and 8b, and the two work rollers 1 and 2, so that the wire can be taken up from the feed reel 4 to the collecting reel 5. In addition, the running pulley wheels 7a and 7b are alternately moved up and down so that the wire 3 will reciprocate.
When a workpiece 10 is moved upward (in the Z-direction) so as to abut the tension-stretched single wire which, while being soaked with an abrasive liquid, is made to run lengthways at a high speed from the feed reel 4 side toward the collecting reel 5 side, the workpiece 10 is simultaneously cut into a plurality of sliced pieces 10a, 10a. . . .
For the multi-wire saws, there are two types of running methods for the wire 3, the first one is a one-way running type and the other is a reciprocating running type.
In the case of the one-way running type, the wire 3 is made to constantly run at a high speed from the feed reel 4 side toward the collecting reel 5 side. Hence, the rate of feeding the wire 3 coincides with the running speed of it at any time.
In contrast, consider a case of the reciprocating running type, where 4 m, for example, of the wire 3 is delivered from the feed reel 4 side to the collecting reel side 5 thereafter 3.5 m, for example, of the wire is returned from the collecting reel 5 side to the feed reel 4 side and this cycle is repeated in a period of 2 sec, for example. The running speed of the wire 3 in this case will be 15 m/min. The running speed of the wire 3 which determines the machining performance is determined by the angular acceleration of the work rollers 1 and 2, and can be set at 200 m/min or more, for example. That is, in the case of the reciprocating running type, the rate of feeding the wire 3 will not necessarily coincide with the running speed of it.
Here, consider a case in which the workpiece 10 to be cut has a cylindrical shape, for example. That is, consider a workpiece having a circular cut face or a cut face whose contact length with the wire 3 varies in the process of cutting.
In this case, the workpiece 10 is worn away and cut in the presence of the abrasive liquid. At the same time, the wire 3 is also worn away by the abrasive liquid. Accordingly, the diameter of the wire 3 on the collecting reel 5 side becomes smaller than that on the feed reel 4 side. In this case, if the pitch of the spiral grooves 11 and 21 formed around the work roller 1 and 2 is constant, the sliced cylinder pieces gradually becomes thicker toward the collecting reel 5 side (toward the upper side in FIG. 2). In order to compensate for the cylinder pices gradually becoming thicker, the distances between the grooves around the work rollers 1 and 2, (namely, p1, p2, . . . , p240; here p1 is on the feed reel 4 side and p240 is on the collecting reel 5 side) are designed so that p1 &gt;p2 &gt;. . . &gt;p240.
FIGS. 3A to 3F are views showing the variation in the diameter of the wire 3 during the course of the cutting and the change of the thickness of the sliced pieces of the workpiece 10 when the cylindrical workpiece 10 is machined. In each of the figures, the left figure portion is a sectional view taken on the line 51-52 shown in FIG. 2 and the right figure portion is a sectional view taken on the line 61-62 shown in the left figure portion. In the above cases, FIGS. 3A and 3B show the situation immediately after the start of cutting; FIGS. 3C and 3D show the situation in the middle of cutting; and FIGS. 3E and 3F show the situation when the cutting is almost finished.
Because of the modification of the pitch of the grooves formed around the work rollers 1 and 2, the thickness of all the sliced pieces 10a, 10a. . . become equal to one another at the same level. However, as each of the sliced pieces (to be referred to as cylinder pieces, hereinbelow) 10a, 10a . . . are observed, the thickness is uneven within the cut surface, and the sliced piece forms a convex shape.
That is, when viewed from the direction perpendicular to the direction of cutting, the cylinder pieces 10a, 10a . . . have the greatest thickness at their center (designated at 101) and this thickness becomes thinner towards both upper and lower edges (102 and 103). This occurs because the contact length (l.sub.X) of the wire 3 with the workpiece 10 immediately after the start of machining and immediately before the end of machining, is at its shortest. This means that the wire 3 will be abraded to a less degree (therefore, the amount of the waste removed becomes greater). In contrast, in the middle of machining, the contact length l.sub.X of the wire 3 with the workpiece 10 becomes maximum so that the wire 3 will be worn away to the greatest degree (therefore, the amount of the waste removed will become small).
This phenomenon is not peculiar to the reciprocating running type but occurs in both the one-way running type and the reciprocating running type. That is, since the cutting by the multi-wire saw is performed by the principle similar to lapping, the running speed of the wire is normally high enough compared to the speed of pushing up the workpiece 10 (if, for example, the speed of wire feeding is 15.6 m/min and the speed of pushing up the workpiece 10 is 5 mm/h, the speed of wire feeding is 18,720 times the pushing-up speed). Further, since also in the reciprocating running type, fresh wire is supplied constantly, the diameter of the wire 3 also varies in the same manner as that in the one-way running type.
Thus, in the conventional multi-wire saw, by the adjustment of the wire pitch as stated above (p1&gt;p2&gt;. . . &gt;p240), the thickness of the sliced cylinder pieces 10a, 10a. . . becomes equal at the same cutting position.
However, when the cylinder pieces 10a , 10a. . . are observed individually, the thickness of each cylinder piece within its cut surface is uneven. In order to prevent the variation of the thickness of the cylinder pieces 10a, 10a. . . due to the ununiformity of the thickness within the cut surface, it is necessary to raise the feeding speed of wire to such a degree that the abraded amount of the wire will be negligible. This means a great increase of the running cost of the machining process.
Even when there is no need to restrict the variation of the thickness within the cut surface of each of the cylinder pieces 10a, 10a. . . , there is still the problem of the wire becoming too abraded. The extert of abrading might cause the wire to snap easily or cause some other problem in usage. The abrasion of the wire was also an inhibiting factor against the improvement of the yield which can be performed by reducing the amount of waste removed.